Design Patterns Basics

Design Patterns: Blueprints for Scalable Code

Design patterns are reusable solutions to common problems in software design. They are not code you can copy-paste, but rather a description of how to structure your classes and objects to make your code more maintainable, flexible, and scalable.

1. Singleton Pattern

The Singleton ensures that a class has only one instance and provides a global point of access to it.

• The Problem: You have a shared resource, such as a database connection pool or a configuration manager, and you don't want to create hundreds of instances of it (which would be wasteful and lead to synchronization issues).

• Implementation: The class makes its constructor private and provides a getInstance() method that returns the existing instance if it exists, or creates it if it doesn't.

• Caution: Singletons can introduce hidden dependencies and make unit testing difficult because they create global state.

2. Factory Pattern

The Factory pattern provides an interface for creating objects in a superclass but allows subclasses to alter the type of objects that will be created.

• The Problem: You have a complex object creation logic. You don't want your main code to depend on the concrete classes of the objects it creates.

• Implementation: You define a "Factory" class or method that handles the instantiation. Instead of using new Service(), you call factory.createService().

• The Benefit: Decoupling. If you need to change the implementation of a service, you only change the factory; your application code remains untouched.

3. Observer Pattern

The Observer pattern defines a one-to-many dependency between objects so that when one object (the "Subject") changes state, all its dependents ("Observers") are notified and updated automatically.

• The Problem: You have a system where multiple parts of the application need to react to a specific event (e.g., a "User Logged In" event).

• Implementation: The Subject maintains a list of Observers. When an event occurs, it iterates through the list and calls an update() method on each one.

• Use Case: This is the foundation of Event-Driven Architecture and UI frameworks (like React or Vue, where the UI "observes" the state).

4. Strategy Pattern

The Strategy pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. It lets the algorithm vary independently from the clients that use it.

• The Problem: You have a class that performs an action in different ways (e.g., a PaymentProcessor that supports Credit Card, PayPal, or Crypto). Without the strategy pattern, you would end up with a massive if-else or switch statement that is hard to maintain.

• Implementation: You define an interface for the algorithm. You then create concrete classes for each strategy (e.g., CreditCardStrategy, PaypalStrategy). The PaymentProcessor takes the strategy as an argument and executes it.

• The Benefit: Open/Closed Principle—you can add a new payment method without changing the existing PaymentProcessor code.

Design Pattern Reference Matrix